This invention relates to the manufacture of the printed circuit boards which are used in many electronic applications. More particularly, the invention relates to an improved solder mask for such circuit boards which employs epoxy resins or other materials having superior physical and electrical performance. The solder mask is applied by a unique method which uses a coated copper foil.
Solder mask (also called "solder resist") is defined as a coating used to protect (mask) certain areas of a primed circuit board during the soldering of connections to the circuit board. The functions of a solder resist are summarized in Primed Circuits Handbook, Clyde F. Coombs Jr., editor, 3rd ed. McGraw-Hill, 1988. Since the solder mask covers most of the other surfaces of the circuit board, it serves to protect the circuitry and to provide electrical insulation. Because much of the printed circuit is covered, only a small area of the circuit is exposed to solder. Therefore, less solder is used and the possibility of solder bridging over the circuit lines and other features is reduced. Also, the transfer of contaminates into the solder container from the circuit board is minimized. Finally, the solder mask reduces dendritic growth of the copper elements on the circuit board.
The present invention is directed to improved permanent solder masks, which remain on the circuit board when in use. In contrast, temporary solder masks are applied and later removed during the circuit board manufacturing process. Two general types of permanent solder masks are recognized--liquid and dry film types. Dry films are placed on the circuit board and while their performance is superior in many respects to films which are applied in liquid form, they are more expensive and, being solid films, they may not achieve full contact with both the copper circuit features and the underlying insulating substrate (e.g., a glass reinforced epoxy resin). Resists applied as liquids, such as by screen printing and the like make full contact with the circuit features and the board but they may fill holes rather than cover them and they are not as capable of precisely discriminating the circuit features to be exposed to solder. Generally, liquid films do not provide a uniform thickness and consequently, the electrical resistance also is not as uniform as would be desired.
A problem common to both types of conventional solder mask is that they generally have a low glass transition temperature (Tg) and they are not as robust as the typical epoxy resins used for the circuit boards. Also, they often are not flame resistant, so that greater amounts of flame retardents are needed in the circuit board to meet Underwriters' Laboratories standards. Ideally, conventional solder mask materials should be replaced with different materials having a higher Tg, better chemical and thermal resistance, and improved flame resistance, and which are easier to process. The inventor has sought and found an improved method of applying a solder mask, which is described below.